Build warning about Android NDK version

[tobine]

Environment information:

• AGI version: Latest master
• Host OS: Windows

Bug description
When building on Windows I see the warning:

WARNING: The major revision of the Android NDK referenced by android_ndk_repository rule 'androidndk' is 21. The major revisions supported by Bazel are [10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]. Bazel will attempt to treat the NDK as if it was r20. This may cause compilation and linkage problems. Please download a supported NDK version.

This contradicts the build documentation that states:

The minimum required version of the NDK is r21.

Reproduction steps
Steps to reproduce the behavior:
Pull latest code and build according to the instructions

[hevrard]

NDK r21 is not yet supported in Bazel, see: bazelbuild/bazel#10811

Still, we're using it to have access to the unified Khronos validation layer. We never had an NDK-related issue after the update to r21, so I think it's safe to ignore this warning.

What is left to do is to ass to our build documentation that this warning is expected, I'll take care of that.

[hevrard]

Fixed by: #307